Petroski on Engineering: Design Begets Design

New designs have a variety of origins. Some are prompted by necessity, some by desire, and some by just plain playfulness.

An interesting historical example that incorporates all three forms of prompting is the Forth Bridge, which was designed and constructed in the 1880s and which still carries railroad trains high above the River Forth near Edinburgh, Scotland.

The Forth Bridge and another across the River Tay about 35 miles north at Dundee had their origins in the need for fixed crossings over the respective rivers so that the North British Railway could do away with the use of ferries to transfer rolling stock wherever the rails came to a river bank. By bridging the two rivers, the railway company could reduce travel time between Edinburgh and Dundee, and so better compete with the inland Caledonian Railway.

The commission to design the two bridges was given to veteran engineer Thomas Bouch. Both river crossings were to be made at their wide estuaries, which the Scots call "firths," and so long bridges were required. The Firth of Tay is shallow, however, and so Bouch designed a bridge with many piers connected by relatively short girders, except for the so-called "high girders" over the navigation channels. When the Tay Bridge opened in 1878, it was the longest in length of any bridge in the world, reaching across almost two miles of water.

As ordinary a design as it was, the Tay Bridge lasted less than two years. During a storm in December 1879, its high-girder trusses collapsed while a train was passing over them, taking the lives of 75 people. The accident naturally led to an investigation, and a court of inquiry found that the structure had been "badly designed, badly constructed, and badly maintained," and engineer Bouch, who had been knighted when the bridge had been completed, was discredited and the commission to design a span over the Forth was taken from him.

A new Tay Bridge, designed by railway engineer William Henry Barlow, was built right beside the failed one, with the stubs of the piers remaining exposed to this day as a reminder of the tragedy. The commission to design a bridge across the Forth was given to the engineering firm of Sir William Fowler and his young partner, Benjamin Baker. There was no confidence in the suspension bridge that Bouch had designed for the Forth, and the railway desired that Fowler and Baker come up with a novel design that would in no way remind potential rail passengers of either of Bouch's designs.

Baker was the lead designer for the new bridge across the Forth, and he came up with a structure based on the cantilever principle. Like a cantilever beam that is supported at one end only, Baker's bridge reached out from its towers without any need for additional support from below. This was an ideal choice for the site, since the Firth of Forth is deep and so erecting a supporting structure in it would be time consuming, dangerous, and expensive. Only three sets of deep foundations, over 1,700 feet apart and supporting 300-foot-high piers, had to be constructed in the water. The daring configuration would give the Forth Bridge the longest spans in the world, a record that would not be surpassed for almost three decades.

The story of that demonstration of the concept of the bridge is really interesting. Before the days of electronics to be able to get such a concept across was a difficult proposition. The solution is very good and filled the bill well.

This is definitely a great way to illustrate concepts of stress and strain! I'll have to keep this technique in mind when trying to explain mechanical problems. By the way, here is a picture (from Wikipedia):

What I really like about the human model described here is that the humans could feel the tensile and compressive forces, rather than just imagine them. Seems like it would be a great exercise for engineering students.

Using words to let us paint our own mental picture is to me the essence of teaching. Teach us to read and imagine, then present a problem and describe a solution and let our minds create the picture and fill in the details. I read the article and had a pretty good concept of what was being described, when I saw the photo, it was obvious what had been described and the details clicked immediately into place. I think those of us who learned to read before there was television may be luckier than our children who had all the solutions presented visually before they developed the ability to imagine. I always enjoy your articles Professor Petroski.

Ken, You are absolutely correct. A picture is worth a lot of words, but it was my understanding that my column was not to be illustrated. Thanks to Mr. Palmer for inserting the classic photo into his comment. HP

This article may not be illustrated, but "Engineers of Dreams" is, and I remember the image shown below is in that book. I've kept it on my shelf waiting for my son to be old enough to understand and appreciate it as he heads towards an engineering career.

Sorry, Mr. P, I had no idea there was any such limitation, it seems I see photo's here quite regularly. Editor?

Bob from Maine, I'm quite proud of my ability to describe things accurately, but like most engineers regardless their artistic ability, I am always sketching stuff during discussions. One wouldn't commit schematics or drawings to a written description. Imaginations are way too variable to assure our minds are on the same page.

I'd seen this fantastic photo some time ago too, (Perhaps in Mr. P's book) and although it all sounded familiar, I still didn't put it together.

An interesting title, although my experience is that designs build on designs. It is often easier to imagine an improvement of some kind when looking at a design then when looking at a blank sheet. Even an unworkable dsign can serve as a basis for ides to produce a much better design. At least that is often what I see. Of course, sometimes we can only guess about what the main goal of a design was. Some designs seek to minimize the required accuracy of components, others strive for the maximum robustness, and it is obvious that many designs are optimized for minimum initial cost, with little attention given to other variables.

So every design can serve as a basis for additional designs, optimized for some particular variable parameter.

Design does beget design. In the article, the design of the demo was a result of the design of the bridge. That is the circle of life of engineering. If you design an end item, there are many other designs that are necessary to make your assembly a reality. If the component requires plastic parts, someone has to design the molds for the part. Someone else handles the design of the molding mahcine to run the molds. This goes on up and down the chain keeping engineers in business.

The comment about secondary required designs brings out the old comment "Nothing is ever simple". The truth is that usually an over-all design does require a lot of little designs, such as nuts and bolts. The good news is thgat we don't need to design those parts new each time.

Dr. Petroski--absolutely fascinating article. I love these stories that give background and weave into the narrative personal history regarding the engineer(s) doing the work. We sometimes forget that many many great engineering designs were accomplished with slide rules, pencils and erasers. (Big erasers at that.) Really demonstrates how far we have progressed with technology. I wonder where we will be fifty years from now. Again, many thanks.

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